This invention relates to a method for producing a valve seat for a cylinder head of an internal combustion engine of the type where an added material is melted to the base material of the cylinder head by a laser beam at the place where the valve seat is to be formed
A method of this type is disclosed in EP 00 92 683 B1.
In the formation of the valve seat, the base material of the cylinder head is made substantially of aluminum. The added material for the formation of the valve seat is either iron or nickel or an alloy containing one of these two metals as the chief ingredient
In this method, however, the problem arises that iron and nickel have a substantially higher melting point than the aluminum of the cylinder head. When a laser beam is applied, therefore, the cylinder head is already fused when the added material first begins to melt. Moreover, it also can happen that previously molten iron has solidified while the aluminum is still molten. This leads to the formation of intermetallic phases in the boundary area between iron and aluminum, resulting in a very brittle structure. For these reasons it is very difficult to achieve a homogeneous bond between the valve seat being created and the base material of the cylinder head, and here too the different surface tensions of the materials play a great role. In the use of nickel, furthermore, considerable recycling problems are involved.
In EP 02 28 282 B1 a cylinder head is disclosed, which consists of an aluminum alloy and in which the valve seat is formed from a plated copper alloy layer. When copper is used for valve seats, however, especially in the case of Diesel internal combustion engines, the problem arises that the sulfur contained in the Diesel fuel attacks the copper, so that problems develop regarding the formation of exhaust gas and corrosion. The use of copper for valve seats is practical therefore only for Otto cycle internal combustion engines and therefore cannot be used in an economical manner.
It is therefore a primary object of the present invention to provide a method for the production of a valve seat for a cylinder head of an internal combustion engine by means of which a valve seat can be produced which has on the one hand sufficient hardness and strength characteristics and on the other hand is capable of entering into a securely adherent bond with the material of the cylinder head, without the formation of pores and fissures in the valve seat.
This object is accomplished according to the present invention by using an alloy or a mixture of an aluminum-silicon alloy and nickel as well as at least one additional alloy component as the added material.
By the use of the alloy according to the invention or of the mixture of an aluminum-silicon alloy and nickel according to the invention as the added material for the formation of the valve seat, a method that is very easy to master is the result, since the properties of this added material are very similar to those of the base material of the cylinder head. The bonding of the valve seat to the cylinder head is achieved by the method of the invention in an extremely satisfactory manner, and especially no air inclusions and fissures develop in the valve seat, so that the process is highly reliable.
By the admixture of nickel and additional alloying components a valve seat is developed which can fully satisfy the requirements of hardness and compressive strength.
In advantageous embodiments of the invention the added material can be an alloy or a mixture of AlSi3O, nickel, iron and boron. This alloy or mixture gives the best results in regard to the requirements to be satisfied by the added material and by the valve seat formed thereby.